Method and system for conveying multiple calls on a single telephone line

ABSTRACT

A system for implementing multiple communication channels on a single twisted pair transmission line is disclosed. The system derives additional communication channels by way of a separate transceiver unit for each derived line. Each transceiver unit communicates in a separate predetermined frequency band. Each transceiver unit, upon connection to the transmission line, automatically utilizes the lowest unoccupied frequency band by monitoring each frequency band for the presence of signal poser. Thus, as many derived lines as will be supported by the customer transmission loop can be readily added.

TECHNICAL FIELD

[0001] This invention relates in general to data communication across atelephone transmission line and, more particularly, to a method andsystem for simultaneously conveying multiple data transmissions over asingle telephone transmission line.

BACKGROUND OF THE INVENTION

[0002] There is an increasing demand on local telephone companies toprovide more communication channels at each customer premises. In anindividual home, for example, there may be a need for two or morecommunication channels to carry voice data and one or more channels tosupport digital communications for such devices as a facsimile machine,a personal computer, or an internet terminal. Various equipment existstoday to enable multiple communication signals to be conveyed on asingle twisted paired telephone transmission line such as digital addedmain line (DAML) systems, basic rate integrated services digital network(ISDN) systems, circuit multiplexers, and some implementations of IPtelephony wherein voice signals are conveyed via Internet ProtocolPackets routed through the Internet. In such existing systems, all ofthe communication channels are terminated by one transceiver-multiplexerat each end of the telephone transmission line, and all of thecommunication channels are combined by a common unit and transmitted asone modulated signal.

[0003] Recently, twisted pair telephone transmission line connectionshave been used for communicating two simultaneous channels such asdigital data and analog voice signals. Typically, a high speed digitalsubscriber line (DSL) channel such as ADSL and a plain old telephonesystem (POTS) channel are established over a single twisted pair wireconnection. A POTS splitter is typically utilized to decouple thechannels into separate frequency bands. The POTS channel usually residesin a frequency spectrum of about 0 kHz to about 4 kHz, and the ADSLchannel resides in a frequency spectrum of about 20 kHz to about 500kHz. A low pass filter is often included in such a system to isolate thechannels and minimize high frequency transients produced byon-hook/off-hook transitions which can degrade the high speed datatransmission on the ADSL channel.

[0004]FIGS. 1A and 1B show one implementation of a DAML system justdescribed. In FIG. 1A, the customer premises 10 is connected to thepublic switched telephone network 12 through the twisted pairtransmission line 14 connected into the main distribution frame 16. DAMLunit 20 is connected to the network interface device 18 through walljack 19. The DAML unit 20 supports two independent communicationchannels 22, 24 by multiplexing the signals and transmitting them acrosstransmission line 14 as a single modulated signal. Phone 1 communicateson baseband POTS. A low pass filter (LPF) 21 isolates the higherfrequency transients and interference between the two communicationchannels. A corresponding DAML unit 13 and LPF 15 are connected on thenetwork side of the system. FIG. 1B represents the frequency band of thesignal transmitted across transmission line 14. Phone 1 communication isbaseband POTS 23 and phones 2 and 3 communicate in a combined, higherfrequency channel 25. Most DAML systems currently omit the baseband POTSchannel.

[0005] Traditional DAML systems are designed to work on nearly allcustomer transmission loops. Since transmission signal quality isrelated to the customer distance from the central office, the number ofcommunication channels a DAML system can support is limited by theworst-case scenario transmission loop in the overall system. In otherwords, the DAML system must be able to support the same number ofadditional communication lines for customers furthest from the centraloffice as it does for customers nearest to the central office.Accordingly, the upper frequency range supported by the DAML units isartificially limited for customers whose transmission loops wouldsupport higher frequency ranges and, therefore, additional communicationchannels.

[0006] The present invention overcomes this drawback by derivingadditional communication channels wherein each additional communicationschannel is modulated into a separate signal in a separate frequency bandby way of a separate transceiver unit such as a DAML. For eachadditional communications channel desired, a separate transceiver unitis connected to the telephone transmission line at the customer premisesin, for example, a wall jack. Each transceiver unit automaticallyutilizes the lowest unoccupied frequency band by monitoring eachfrequency band for the presence of signal power. This configurationenables transmission lines of customers closer to the central office tosupport several derived communications channels, whereas transmissionlines for customers further from the central office with less usablebandwidth could still be used to support a fewer number of derivedcommunications channels.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] For a more complete understanding of the invention, referenceshould now be had to the embodiments illustrated in greater detail inthe accompanying drawings and described below by way of examples of theinvention. In the drawings:

[0008]FIG. 1A is a schematic diagram of prior art DAML implementation;

[0009]FIG. 1B is a graph of the frequency bands associated with the DAMLimplementation of FIG. 1A;

[0010]FIG. 2A is a schematic diagram of one embodiment of the presentinvention for adding additional communications channels to a singletelephone line; and

[0011]FIG. 2A is a graph of the frequency bands associated with theembodiment illustrated in FIG. 2A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0012] Referring to FIG. 2A, there is shown a schematic block diagram ofone configuration of the present invention for adding additionalcommunication channels to a single twisted pair telephone transmissionline. In FIG. 2A, the customer premises 30 is connected to the maindistribution frame 32 of the public switched telephone network 34 bytwisted pair transmission line 36. The transmission line 36 isphysically connected to the customer premises 30 at the networkinterface device 38. The transmission line 36 terminates at variouslocations within the customer premises 30 at a series of commontelephone wall jacks 40-44. A plurality of transceiver/multiplexer unitsare connected into the wall jacks 40-44. These include phone A 46, phoneB 48, phone C 50, PC modem 52, and integrated DAML unit and phone 54.Between phone C 50 and wall jack 42, and between PC modem 52 and walljack 43, are DAML units 56 and 58, respectively. Similarly, betweenphone A 46 and wall jack 40, and between phone B 48 and wall jack 41,are low pass filters 60 and 62, respectively.

[0013] The network-side of the transmission line 36 includes DAML unit64 and low pass filter 66. The MDF 32 connects the telephone line 36 tothe DAML unit 64 and the LPF 66. The LPF 66 extracts the 0-4 kHz bandthat carries channel 1. This is connected via the MDF 32 to the PSTN 34.The DAML unit 64 modulates and demodulates the voice signals that areconnected, via the MDF 32, to the PSTN 34. The PSTN 34 switches each ofthe voice signals from the DAML unit 64 and LPF 66 as traditionalcircuit switched voice calls without any special actions required of thePSTN 34 due to the use of the DAML 64.

[0014]FIG. 2B represents a graph of the frequency bands associated witheach of the communication channels of the configuration of FIG. 2A. InFIG. 2A, phones A and B 46, 48 transmit voice signals as a basebandanalog signal on line 1, which is represented in FIG. 2B by POTS channel1. This frequency band would typically have a range from 0 Hz toapproximately 4 kHz. Derived lines 2, 3, and 4 use digital modulationtransmission, wherein the signals for lines 2, 3, and 4 are modulatedinto distinct frequency bands which are represented in FIG. 2B. Phones Aand B 46, 48, represent extensions on the same line, thus their signalsare superimposed into the same channel 1. Also, phones A and B 46, 48,are powered from the central office via transmission line 36, whereasthe transmission units for the derived lines 2, 3, and 4 are typicallypowered from a power source at the customer premises 30 such ascommercial AC power.

[0015] Because physically separate transceiver units are used to deriveeach additional communications channel, derived lines can be easilyplaced in separate rooms within the customer premises 30, and additionalderived lines can be readily added. This also allows the same type oftransceiver equipment to be used regardless of the number of derivedlines. These derived lines, or communications channels, can be used forvoice, facsimile, or data transmissions such as, for example, PC accessto the Internet. The number of derived lines can be one or more, and asingle transceiver unit may derive more than one line.

[0016] The transceiver units used to derive additional communicationschannels are preferably implemented using ADSL-lite (as described in ITURecommendation G.992.2) or splitterless ADSL technology. This providesthe benefit of a customer-end transceiver-multiplexer that is easilyinstalled by the customer within the premises 30 without requiring thecost and inconvenience of a premises installation visit by a telephonecompany technician. Although the preferred location for the DAML unit iswithin the customer premises 30, they could also be deployed at a siteoutside the premises 30 as part of the telephone company's network.

[0017] As mentioned above, low pass filters 60, 62 are included toisolate the higher frequency derived communications channels fromon-hook/off-hook transients created by phones A and B 46, 48. The LPF60, 62 is typically located at the wire connecting the phone to the walljack.

[0018] The method of deriving additional communications channels willnow be described by way of example with reference to FIGS. 2A and 2B.Assume that there is one communications channel at customer premises 30and that phone A 46 and phone B 48 represent extensions on that samecommunications channel (line 1). To add an additional communicationschannel such as phone C on line 2, a DAML unit 56 is connected to thewall jack 42 at the customer premises 30. In addition, low pass filters60 and 62 will preferably be added between phones A and B and wall jacks40 and 41 to isolate the higher frequency communications channel of line2. The DAML unit 56 is configured to recognize discrete frequency bandsabove the POTS channel (line 1). These frequency bands are predefined atthe time of the unit's manufacture. For example, frequency band 2 couldbe defined as 40-60 kHz, frequency band 3 could be defined as 70-90 kHz,frequency band 4 could be defined as 100-120 kHz, etc. These frequencybands would correspond to the frequency bands represented in the graphof FIG. 23 for lines 2, 3 and 4.

[0019] Upon connection to the wall jack 42, DAML unit 56 observes thesignal energy in each of the defined frequency bands starting with thelowest. The DAML unit 56 utilizes the lowest frequency band for whichthe observed signal power is less than a threshold value whichrepresents the minimal expected signal power observed for a frequencyband in use by another unit. In this example, DAML unit 56 would likelytransmit signals in frequency band 2 since no additional DAML units areconnected to the transmission line 36 at this time. Phone C would thencommunicate over line 2 through DAML unit 56.

[0020] To derive additional communication channels (lines 3 and 4),additional DAML units 54 and 58, are connected to the transmission line36 through wall jacks 44 and 43, respectively. DAML unit 54 is shown asan integrated telephone and DAML unit. Such a unit could have a reducedcost and simplify the installation processor by reducing the number ofcomponents to interconnect. In addition, line 3, as shown in FIG. 2A isused to support PC data transmission. Of course, phone C 50, PC 52 andintegrated telephone unit 54 are merely illustrative of digitalcommunications devices and could be substituted for any such device.Additional derived lines can be added in a similar manner so long as theusable bandwidth on the customer transmission loop supports such lines.Hence, customers located closer to the telephone company's centraloffice would likely be able to support more derived communication linesthan customers located further from the central office because increaseddistance typically reduces the usable bandwidth of a customertransmission loop.

[0021] Upon connection, each DAML unit observes the signal energy ineach of the defined frequency bands. The signal energy in each of thefrequency bands is monitored by the use of a fast Fourier transformalgorithm implemented in firmware on a digital signal process (DSP)integrated circuit located within each DAML unit. The signal powerwithin each predefined frequency band is integrated across the frequencyband and averaged over time. The DAML unit utilizes the lowest frequencyband for which the observed signal power is less than a threshold valuethat represents the minimum expected signal power observed for afrequency band in use by another unit. To minimize the probability ofcontention in the event that several DAML units on a line attempt tostart up simultaneously, each DAML unit preferably monitors the signalenergy in a frequency band for a bounded random duration of time. Oncethe DAML unit has found a frequency band with no apparent signal power,it transmits its signal in that frequency band by use of a pass bandmodulation method. For example, quadrature amplitude modulation (QAM)with a carrier placed at the center of the chosen frequency band. Toreduce interference between frequency bands, filtering is implemented byway of the DSP. Interference can be further reduced by placing an emptyguard band between each of the defined frequency bands.

[0022] As an alternative embodiment, each customer line can beoversubscribed. In other words, the number of communication channels canexceed the number of available frequency bands provided that only asmany transceiver units as there are frequency bands are transmitting oroff-hook at any given time. In addition, in the event that a DAML unitdetermined that its signal transmission quality as measured by thesignal-to-noise ratio or bit error rate was unacceptable, the DAML unitwould stop transmission and search for another acceptable frequencyband.

[0023] With regard to the transceiver units, if higher data rates ormultiple derived phone lines are required of a single transceiver unit,multiple frequency bands, preferably adjacent, would be utilized. Thus,applications such as video can be supported by combining frequencychannels.

[0024] In another embodiment, one telephone transmission line can beused to support more than one customer premises. In this scenario, thesingle twisted pair telephone transmission line is connected to multiplecustomer sites wherein transceiver units such as those described withreference to FIG. 2A are used at each customer site to createcommunications channels in separate distinct frequency bands. In such acase, however, it is important that only one customer site use the baseband POTS frequency channel connected through a low pass filter,otherwise a “party line” would result.

[0025] In still another embodiment, the transceiver unit 64 at thecentral office could be divided into separate transceiver units for eachderived line.

[0026] While the invention has been described in connection with one ormore embodiments, it is to be understood that the invention is notlimited to these embodiments. On the contrary, the invention covers allalternatives, modifications, and equivalents as may be included withinthe scope and spirit of the appended claims.

What is claimed is:
 1. A method of implementing a plurality ofcommunication channels on a single twisted pair telephone connectioncomprising the steps of: interfacing a first communication device withsaid telephone connection, said first communication device configured tocommunicate over said telephone connection on a first channel defined bya first frequency band; interfacing a second communication device withsaid telephone connection, said second communication device configuredto communicate with said telephone connection on a second channel; andinterfacing a third communication device with said telephone connection,said third communication device configured to communicate with saidtelephone connection on a third channel, wherein said first, second, andthird communication channels each reside in separate predeterminedfrequency bands.
 2. The method of claim 1 wherein the step ofinterfacing said second communication device includes the step of:monitoring the signal power of a second frequency band above said firstfrequency band and, if the detected signal power is below apredetermined level, then assigning said second channel to said secondfrequency band, else monitoring the signal power of a third frequencyband above said first and second frequency bands and, if the detectedsignal power is below a predetermined level, then assigning said secondchannel to said third frequency band.
 3. The method of claim 2 whereinthe step of interfacing said third communication device includes thestep of: monitoring the signal power of a second frequency band abovesaid first frequency band and, if the detected signal power is below apredetermined level, then assigning said third channel to said secondfrequency band, else monitoring the signal power of a third frequencyband above said first and second frequency bands and, if the detectedsignal power is below a predetermined level, then assigning said thirdchannel to said third frequency band.
 4. The method of claim 3 whereinthe steps of monitoring the signal power of said second and thirdfrequency bands includes the step of integrating and averaging themeasured signal power over a predetermined time period.
 5. In a networkconnection including a first communication device communicating withsaid network across a single twisted pair telephone line in the basebandPOTS frequency band, a method of deriving additional communicationchannels over said single twisted pair telephone line comprising thesteps of: coupling a plurality communication devices to said networkconnection; detecting a communication request from one of saidcommunication devices and, in response; monitoring the signal quality ina series of predefined frequency bands successively higher than saidPOTS frequency band and configuring said communication device tocommunicate with said network across the first said predefined frequencyband wherein said signal quality is above a threshold value.
 6. Themethod of claim 5 wherein the step of configuring said communicationdevice to communicate with said network across the first said predefinedfrequency band wherein said signal quality is above a threshold valuefurther comprises the step of monitoring the signal quality of saidcommunications with said network and, if the signal quality deterioratesbelow said threshold, monitoring the signal quality in said remainingpredefined frequency bands, and reconfiguring said communication deviceto communicate with said network across the first said predefinedfrequency band wherein said signal quality is above a threshold value.7. The method of claim 5 wherein said first communication device andsaid plurality of communication devices are located at a single customerpremises and communicate with said network via a single twisted pairtelephone line.
 8. The method of claim 5 wherein said firstcommunication device is located at a first customer premises and atleast one of said plurality of communication devices are located at asecond customer premises, said first and second customer premisescommunicating with said network via the same twisted pair telephoneline.
 9. A communications arrangement between a PSTN and a customerpremises connected by a single twisted pair telephone transmission linecomprising: a first communication device configured to communicateacross said telephone transmission line on a first channel defined by afirst frequency band; a second communication device configured tocommunicate across said telephone transmission line on a second channel;and a third communication device configured to communicate across saidtelephone transmission line on a third channel, wherein said first,second, and third communication channels each reside in separatepredefined frequency bands, said second and third communication channelsbeing assigned a respective frequency band in response to a respectivecommunication request from said communication device by sequentiallymonitoring the signal quality in each successively higher frequency bandand configuring said communication device to communicate with saidnetwork across the first said predefined frequency band wherein saidsignal quality is above a threshold value..
 10. The arrangement of claim9 wherein said first communication device is a telephone and said firstfrequency band is the baseband POTS frequency band.
 11. The arrangementof claim 9 wherein said second and third communication devices areconfigured to communicate across said telephone transmission line usingquadrature amplitude modulated digital signals.